The present invention relates to a gas-liquid distribution system for downflow reactors which have one or more fixed beds of solids. Reactors of this type are common in the chemical and petroleum refining industries for catalytic processes such as hydrotreating, hydrocracking, hydrodesulfurization, hydrofinishing, and hydrodewaxing. The distributor system of the present invention is particularly useful for effecting mixed-phase reactions between a liquid and a vapor over a solid catalyst.
Fixed-bed reactors typically contain one or more beds of solid particulate catalyst over which a gas, a liquid, or a gas/liquid mixture passes in a downward flow. Optimal reactor performance is achieved when all catalyst is fully contacted by the process fluid(s).
It is common in fixed-bed reactors to employ multiple solids beds disposed vertically throughout the vessel with injection of gas or liquid between each pair of beds. Interbed injection may be needed to replenish depleted reactants, to quench the process fluids following exothermic reactions, or to introduce a different feed stream. If the beds contain different catalysts, it is possible to stage somewhat different reaction zones within a single vessel. In all cases it is critical to establish good fluid distribution at the top of each catalyst bed.
Distributor design for fixed-bed reactors typically has two objectives. The first is completeness of coverage, which usually involves maximizing the number of points from which the distributor disperses the fluid onto the catalyst. The second is uniformity of coverage, which requires that the amounts of fluid dispersed from each point be equal across the reactor. Of the two, uniformity of coverage is more difficult to achieve because in commercial reactors it is not practically possible to assure perfect levelling of the distributor. Fabrication and installation inaccuracies typically result in variation of 1/8" to 1/2" in distributor elevation across the diameter of a commercial vessel, and variations as large as 3/4" have been measured. Even if perfect leveling could be achieved at the outset, it would likely deteriorate during operation as the reactor internals are subject to thermal expansion and significant static load. Typically distributor design involves accepting a baseline variation in levelness and devising a system to be as insensitive to these variations as possible.
The consequences of poor fluid distribution in fixed-bed reactors can be severe. Poor micro-distribution, that is local dispersion in the area of each distribution point, leads to delayed contacting of reactants, and also to regions of unutilized solids. Poor macro-distribution, that is distribution across the reactor as a whole, leads to lateral temperature gradients, possible phase separation, and deficiency of limiting reactants further down the bed. The overall impact of either type of maldistribution is an apparent loss in catalyst activity, and also a possible failure to meet product specifications. In addition, chronic maldistribution can lead to plugging of part of the solids bed, excessive pressure drop, and premature shutdown.
Many different types of distribution means are known. The simplest ones comprise little more than a pierced or slotted plate. Others have various forms of orifices, weirs, slots, or more complicated devices for promoting the desired uniformity of gas/liquid flow.
U.S. Pat. No. 2,898,292 teaches a distribution means consisting of a plurality of vertical open pipes with notches in the upper rim for liquid overflow. Gas and liquid are introduced onto the catalyst bed at velocities not exceeding 30 feet per second to avoid disturbing the surface of the solids.
U.S. Pat. No. 3,146,189 discloses a distributor tray in which liquid passes onto the solids bed through short pipes, while gas passes through larger and longer pipes which extend downward into the solids bed. This type of distributor is inferior for mixed-phase reactions because it acts to separate the gas and liquid rather than introducing them as a mixture onto the catalyst.
U.S. Pat. No. 3,353,924 provides a gas-liquid distributor consisting of pipes with long vertical slots on the sides so that liquid flow through the distributor increases as liquid level on the tray increases. A simple fluid mechanical analysis of such a device shows that the flow through the pipes varies with the liquid height according to: EQU Q=C.times.h.sup.1.5, (I)
where PA1 where PA1 where
Q=volume flow rate through pipes, PA2 h=height of liquid above bottom of slot, and PA2 C is a constant. PA2 Q=volume flow rate through pipes, PA2 h=height of liquid above bottom of notch, and PA2 C is a constant. PA2 Q=volume flow rate through pipes, PA2 h=height of liquid above the PA2 centerline of hole(s), and PA2 C is a constant.
This behavior is undesirable because the 1.5-power dependence on liquid height makes the distributor very sensitive to variations in levelness. In addition, this device uses separate, larger chimneys for gas flow which restricts the number of liquid irrigation points on the tray.
U.S. Pat. No. 3,524,731 teaches a type of pipe distributor using inverted triangular notches rather than straight-sided slots. This approach results in the flow equation: EQU Q=C.times.h, (II)
Although improved over the 1.5-power dependence of the preceding patent, this device still shows a strong influence of unlevelness. The exact power on liquid height (h) depends on the ratio of altitude to base width of the triangular notches. Moreover, although the liquid flow is normally through the pipes, at high liquid rates liquid also passes by overflow through separate chimneys normally reserved for gas flow. During such operation the rates of liquid flow through the pipes and the chimneys are quite different.
U.S. Pat. No. 3,685,971 provides a pipe distributor with no slots or notches of any type. This is the least effective type of pipe distributor because on an unlevel tray the liquid flow will favor the lowest pipe on the tray almost to the exclusion of the others. The use of any type of slot or notch at the top of the pipe to meter liquid overflow is an improvement over a completely smooth pipe rim.
U.S. Pat. No. 4,126,539 discloses a gas-liquid distributor system having pipe distributors with rectangular notches in the upper rim as well as circular holes between the rim and the tray deck. This patent contemplates that the rectangular notches (weirs) at the top of the pipes define the liquid level on the tray with the circular holes insuring that there is a flow through the pipes of the tray if the liquid level drops below the notches. Thus, this patent contemplates that the distributor system functions at the weirs in a manner similar to the system of U.S. Pat. No. 3,353,924 discussed above.
A distributor of the type disclosed U.S. Pat. No. 4,126,539 can be operated with a liquid height below the rectangular notches in the upper rim of the pipes and above the circular holes. Such operation would be a vast improvement over the above discussed prior art because the bulk of the liquid flow would pass through the holes as a jet which is sheared by the gas passing vertically downward. The shearing action would break up the liquid and would thereby improve gas-liquid contact before the fluids reach the catalyst bed. In this case, the relation between liquid flow and liquid level for the circular holes can be expressed as: EQU Q=C.times.h.sup.0.5, (III)
Of the art discussed here, such use of the distributor of U.S. Pat. No. 4,126,539 would provide a minimization of sensitivity of liquid flow to variations in level, with the rectangular notches being used for abnormally high liquid rates when the full flow cannot be conveyed through the holes. A disadvantage of such use of this distributor would arise at low liquid flow rates which cause the liquid level on the tray to fall between the top and bottom of the holes. Under these conditions the above flow equation III no longer holds, and a 1.5 power dependence on liquid height makes the distributor strongly sensitive to variations in levelness similar to that discussed above with respect to the slotted pipes of U.S. Pat. No. 3,353,924. A low liquid level could be minimized by sizing the circular holes smaller, but hole diameters less than about 1/4" would be impractical due to the possibility of plugging. Thus for a given reactor there is a minimum liquid rate for which downpipes with holes are effective, below which good distribution cannot be guaranteed.
It is known to design gas-liquid distributors with vertical downpipes having holes drilled in the sides to control liquid level on the tray. However, such pipes are fabricated with the same number, size, and location of holes on each, and thus suffer at very low liquid rates from the poor performance noted above.
Although gas-liquid distribution for fixed-bed reactors has been studied and improved upon for many years, it is still common to observe evidence of maldistribution of reactants in commercial reactors. Temperature maldistribution in exothermic processes generally indicates greater fluid flow in one part of the bed versus another. Rapid pressure drop buildup often reveals coking in the bed caused by regions of stagnant flow or insufficient reactants. Fresh (not discolored) catalyst is sometimes found when fixed-bed units are serviced after two to three years in operation, indicating flow bypassing. These findings indicate that at least some aspects of fluid flow in gas-liquid distributors has not been well understood. Yet in the petroleum refining and other industries, public demand and government regulations have dictated the removal of certain compounds from chemical products, necessitating more severe operation and greater need for optimal and reliable reactor performance. Effective distribution in reactors is critical to meeting this demand.